LDS doctrine teaches that those that are mentally disabled are guaranteed entrance into the highest heaven. They are considered the purest of the pure among us.
Mormon afterlife consist of 4+ possibilities:
Hell
Outer darkness: this is the LDS version of hell - its reserved for males who held the priesthood, met Jesus face to face and denied his divinity. (women can not go to hell/outer darkness in LDS doctrine, they can not hold the priesthood which is required to be eligible for outer darkness, which is defined as being out of the light or presence of God)
Heaven
Terrestrial kingdom: murders, thieves etc. who had no belief in Jesus (yes they go to heaven in LDS beliefs).
Telestial kingdom: all those that lead good lives but did not accept the LDS Gospel (Catholics, non-active mormons etc)
Celestial Kingdom: Those that were baptized Mormon and followed the LDS Gospel.
Three kingdoms within the Celestial kingdom (per wikipedia, cause Im too lazy to type it all out!): Joseph Smith taught that the celestial kingdom itself is subdivided into three "heavens or degrees".[9] Only those individuals who are sealed in celestial marriage to a spouse in a temple while alive (or after death by proxy) will be permitted to enter into the highest degree of celestial kingdom.[10] These individuals will eventually become "exalted"[11] and will be permitted to live "the kind of life God lives" as literal gods and goddesses, as Doctrine and Covenants 132 explains.[12] The nature of the other two degrees within the Celestial Kingdom have not been described, except to say that the people who go there will become "ministering angels".[13]